PHOENIX (AP) — The U.S. Justice Department is filing a lawsuit challenging the constitutionality of Arizona’s new law targeting illegal immigrants, setting the stage for a clash between the federal government and state over the nation’s toughest immigration crackdown.

The planned lawsuit was confirmed to The Associated Press by a Justice Department official with knowledge of the plans. The official didn’t want to be identified before a public announcement planned for later Tuesday.

The lawsuit will argue that Arizona’s new measure requiring state and local police to question and possibly arrest illegal immigrants during the enforcement of other laws, like traffic stops, usurps federal authority.

Tuesday’s action has been expected for weeks. President Barack Obama has called the state law misguided. Supporters say it is a reasonable reaction to federal inaction on immigration.

The law requires officers, while enforcing other laws, to question a person’s immigration status if there’s a reasonable suspicion that they are in the country illegally.

Republican Gov. Jan Brewer signed the law in April, and it was set to go into effect July 29. The lawsuit could delay implementation of the law.

Arizona passed the law after years of frustration over problems associated with illegal immigration, including drug trafficking and violent kidnappings. The state is the biggest gateway into the U.S. for illegal immigrants, and is home to an estimated 460,000 illegal immigrants.

The lawsuit is expected to be announced by Attorney General Eric Holder and Homeland Security secretary Janet Napolitano, a former Arizona governor.

President Barack Obama addressed the Arizona law in a speech on immigration reform last week. He touched on one of the major concerns of federal officials, that other states were poised to follow Arizona by crafting their own immigration enforcement laws.

“As other states and localities go their own ways, we face the prospect that different rules for immigration will apply in different parts of the country,” Obama said. “A patchwork of local immigration rules where we all know one clear national standard is needed.”

The law makes it a state crime for legal immigrants to not carry their immigration documents and bans day laborers and people who seek their services from blocking traffic on streets.

The law also prohibits government agencies from having policies that restrict the enforcement of federal immigration law and lets Arizonans file lawsuits against agencies that hinder immigration enforcement.